In recent years, studies have been actively conducted on non-aqueous electrolyte secondary batteries, lithium ion secondary batteries in particular, which provide a high voltage and a high energy density. A non-aqueous electrolyte secondary battery generally employs a transition metal oxide, such as LiCoO2, as the positive electrode active material, a carbon material as the negative electrode active material, and a porous sheet made of polyethylene, polypropylene or the like as the separator.
Generally, a non-aqueous electrolyte contains a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. As the non-aqueous solvent, a cyclic carbonic acid ester, chain carbonic acid ester, cyclic carboxylic acid ester and the like can be used. As the lithium salt, lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4) and the like can be used.
From the viewpoint of further improving the battery characteristics of non-aqueous electrolyte secondary batteries, various improvements have been made to positive electrode active materials, negative electrode active materials, separators, non-aqueous electrolytes, and so on.
For example, Patent Document 1 discloses a proposal in which a porous film containing an inorganic oxide and a binder is carried on the active material layer of an electrode. With this proposal, separation of the active material from the electrode during production of the battery and reattachment of the separated active material to the electrode are suppressed. Accordingly, it is possible to prevent internal short circuiting from occurring in the battery.
Patent Document 2 also discloses a proposal in which a porous film containing an inorganic oxide and a binder is carried on the active material layer of an electrode. The porous film adsorbs free acids produced in the non-aqueous electrolyte. Consequently, the action of the free acids prevents the positive electrode active material from being dissolved, improving the cycle characteristics and storage characteristics of the lithium ion secondary battery.
Patent Document 3 discloses a proposal to use a separator in which a porous sheet and a layer that contains a heat resistant nitrogen-containing aromatic polymer such as aramid and a ceramic powder are laminated. With this proposal, both shut-down properties and heat resistance can be achieved, improving battery safety.    Patent Document 1: Japanese Laid-Open Patent Publication No. Hei 7-220759    Patent Document 2: Japanese Laid-Open Patent Publication No. Hei 10-214640    Patent Document 3: Japanese Laid-Open Patent Publication No. 2000-30686